Abstract. An anomalous climatic event occurred in the Indian Ocean (IO) region during [1997][1998], which coincided with a severe drought in Indonesia and floods in parts of eastern Africa. Cool sea surface temperature anomalies (SSTAs) were present in the eastern IO along and south of the equator. Beginning in July 1997, warm SSTAs appeared in the western IO, and they peaked in February 1998. An ocean general circulation model is employed to investigate the dynamic and thermodynamic processes that caused the SSTAs associated with this and other similar IO events. The eastern cooling resulted from unusually strong upwelling along the equator and Sumatra. The Sumatran upwelling was forced both locally by the stronger alongshore winds and remotely by equatorial and coastal Kelvin waves. By the end of 1997, weakening of the winds and the associated reduction in latent heat loss led to the elimination of the cold SST anomalies in the east.The western warming was initiated by weaker Southwest Monsoon winds and maintained by enhanced precipitation forcing, which resulted in a barrier layer structure. Analysis of the mixed layer temperature equation indicates that a downwelling Rossby wave contribution was crucial for sustaining the warming into February 1998. It is tempting to suppose that the 1997 event was related to the E1 Nifio-Southern Oscillation (ENSO) event that took place in the Pacific at the same time. Indeed, weaker IO events occur quite regularly in the control run that evolve similarly to the 1997 event, and they are often but not always related to ENSO. We speculate that these events represent a natural mode of oscillation in the IO, which is externally forced by ENSO but also excited by ocean-atmosphere interactions internal to the IO. IntroductionAn extreme climatic event took place in the Indian Ocean (IO) during 1997 and early 1998. It was associated with the second-largest sea surface temperature anomalies (SSTAs) occurring in the region during the past half century. Plate la illustrates the time development of observed SSTAs along the equator during the event. In the late spring of 1997 a surface warming ensued over the western region. (Throughout this paper, seasons always refer to those in the Northern Hemisphere.) It peaked in February 1998, by which time it had spread across most of the tropical IO.Strong cooling was evident in the eastern ocean by September 1997, and it continued into January 1998. As a consequence of these changes, there was an equatorial SST gradient from east to west, a reversal of the climatological situation. The wind stress field was also highly unusual during the fall, with easterlies along the equator replacing the climatological west½rli½s (Plate 2a) and strengthened alongshore winds off It is reasonable to suppose that this remarkable IO event was related to the E1Nifio-Southern Oscillation (ENSO) event that took place at the same time in the Pacific. In fact, a number of similar but weaker IO events occurred during the past 40 years that are also related to ENSO (s...

Socioeconomic challenges continue to mount for half a billion residents of central India because of a decline in the total rainfall and a concurrent rise in the magnitude and frequency of extreme rainfall events. Alongside a weakening monsoon circulation, the locally available moisture and the frequency of moisture-laden depressions from the Bay of Bengal have also declined. Here we show that despite these negative trends, there is a threefold increase in widespread extreme rain events over central India during 1950–2015. The rise in these events is due to an increasing variability of the low-level monsoon westerlies over the Arabian Sea, driving surges of moisture supply, leading to extreme rainfall episodes across the entire central subcontinent. The homogeneity of these severe weather events and their association with the ocean temperatures underscores the potential predictability of these events by two-to-three weeks, which offers hope in mitigating their catastrophic impact on life, agriculture and property.

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